1. Field of the Invention
This invention relates to an electrostatic developing toner used in an image-forming apparatus such as a printer, copier or facsimile machine which forms an image by developing an electrostatic latent image formed on a photoconductive layer of a photoconductive drum using a toner, i.e. by an electrophotographic method. In particular, it relates to an electrostatic developing toner which can effectively suppress fogging of the image by setting a ratio (d/D) between a toner average particle diameter D and an average particle diameter d of iron oxide particles contained in the toner which function as a colorant, or by setting a ratio (σr/σs) between a residual magnetization σr and a saturation magnetization σs of the iron oxide particles. It further relates to an electrostatic developing toner which can suppress the cracking amount of the photoconductive layer on the photoconductive drum accompanying the formation of the image even after about 10000 images have been formed.
2. Description of the Related Art
In the past, various image-forming apparatuses have been proposed featuring the formation of an image by an electrophotographic method using an electrostatic developing toner, wherein an additive such as silica particulates is added to toner particles containing a colorant to develop an electrostatic latent image formed on a photoconductive layer of a photoconductive drum.
For example, JP Laid-open Patent Publication No. 05-341556 discloses a toner used in an image-forming apparatus wherein an electrostatic latent image is formed on a photoconductive layer of a latent image carrier via an optical source such as a laser, and toner is supplied to the electrostatic latent image from a toner carrier in contact with the latent image carrier to develop the electrostatic latent image. This toner is a one-component toner containing 20-50 wt % of iron oxide in a binder resin containing a colorant such as carbon black.
JP Laid-open Patent Publication No. 11-143121 discloses a toner used in an image-forming apparatus wherein an electrostatic latent image is formed on a photoconductive layer of an electrostatic latent image carrier, and the electrostatic latent image is developed by supplying toner from a developer carrier (developing roller). This toner contains a magnetic powder having a saturation magnetization σs of 5 A.m2/kg or less, and a residual magnetization σr of 3 A.m2/kg or less.
When magnetic powders such as metal oxides are added to the toner for various purposes such as suppressing image fogging, it is necessary not only to consider the amount of magnetic powder added to the toner, but also the magnetic properties of these magnetic particles such as their saturation magnetization σs and residual magnetization σr.
However, although JP Laid-open Patent Publication No. 05-341556 states that the iron oxide content of the toner is 20-50 wt %, no mention is made of the other magnetic properties of the iron oxide.
JP Laid-open Patent Publication No. 11-143121 discloses that the magnetic powder added to the toner has a saturation magnetization σs of 5 A.m2/kg or less, and a residual magnetization σr of 3 A.m2/kg or less, however as in the case of No. 05-341556, no mention is made of the other magnetic properties of the magnetic powder.
JP Laid-open Patent Publication No. 11-194557 discloses an image-forming apparatus wherein a good image exposure is obtained according to a film pressure of an outermost layer of a photoconductive drum by inputting data relating to the photoconductive drum drive time and the time during which a voltage is applied to the charging roller, together with data relating to the contact pressure of a cleaning blade on a photoconductive drum from a non-volatile memory, calculating a film thickness of the outermost layer of the photoconductive drum based on this data in a control unit, and controlling the image exposure of an exposure apparatus on the photoconductive drum based on the calculated film thickness of the photoconductive drum.
In the image-forming apparatus described in JP Laid-open Patent Publication No. 11-194557, two factors are considered whereby the photoconductive layer formed on the outer circumference of the photoconductive drum may be scraped when the image is formed. The first factor is that a contact charging method is used wherein a charging roller is brought into contact with the photoconductive drum to charge the outer circumferential surface of the photoconductive drum, and the photoconductive layer on the photoconductive drum may be scraped by the charging roller when the image is formed. The other factor is that a residual toner removal method is used wherein a cleaning blade is brought into pressure contact with the photoconductive layer surface on the photoconductive drum to remove residual toner on the photoconductive drum surface after transfer of the toner image to a transfer material, and the photoconductive layer on the photoconductive drum may be scraped by the cleaning blade.
Hence, in the image-forming apparatus disclosed in JP Laid-open Patent Publication No. 11-194557, due to the design of the image-forming apparatus, the scraping of the photoconductive layer by the charging roller and the cleaning blade which are brought into contact with the circumferential surface of the photoconductive drum, are considered.
Due to the design of the image-forming apparatus, if there are members which come into contact with the circumferential surface of the photoconductive layer of the photoconductive drum, the photoconductive layer will be scraped due to the frictional contact between these members and the photoconductive layer, but these are not the only possible factors responsible for the scraping of the photoconductive layer, and it is necessary to consider scraping of the photoconductive layer by various components of the electrostatic developing toner used in the image-forming apparatus.
For example, if the colorant contained in the toner particles of the electrostatic latent image toner is a particulate pigment, its particle size and amount in the toner must be considered as possible factors in the scraping of the photoconductive layer, and if silica particulates are added to the toner particles, their particle size and addition amounts must also be considered.